This invention relates to oxygen steelmaking.
Oxygen steelmaking is one of the main building blocks of a modem industrial economy. There are a range of different oxygen steelmaking processes, including the open hearth process, bottom blown steelmaking processes, including the Bessemer and Q-BOP processes, top blown converter processes including the LD-AC process (also known as BOS (basic oxygen steelmaking) or BOP (basic oxygen process)), and electric arc furnace processes which include the step of lancing molten ferrous metal with oxygen.
High purity oxygen (i.e. oxygen having a purity of greater than 99%) has always been used in such oxygen steelmaking processes because nitrogen has a deleterious effect on steel. In order to form the high purity oxygen air is separated by rectification. The composition of air is such that in order to achieve such purity levels, a rectification column from which the oxygen product is taken has a section for separating argon from oxygen. Since argon and oxygen have similar volatilities to one another, this section contributes substantially to the total height of the rectification column and to the total energy requirements of the air separation process.
It is a general aim of modern air separation processes to minimise the power consumption of the air separation plant without adding unduly to its capital cost.
It is sometimes required that the same air separation plant should supply both a steelmaking and an iron making process. The iron making process may employ a conventional blast furnace which is fed with a charge comprising iron oxide, coke and at least one slagging or fluxing agent, or be a so-called direct reduction process in which coal is used instead of coke. In a direction reduction process, the charge is fed to a reduction vessel in which the iron ore is reduced by a gas mixture comprising hydrogen or carbon monoxide and the resulting iron is melted in a second vessel in which the coal is simultaneously gasified so as to produce the reducing gas mixture. 95% pure oxygen is typically used for the gasification of the coal. As a result, the capital and operating costs of the air separation plant are less when an oxygen product of greater than 99% purity is also required. Once, however, there is this requirement to produce such a pure oxygen product, there is a need to reboil the high purity liquid oxygen at the bottom of the rectification column from which the oxygen products are taken. The higher the purity of the oxygen, the higher is its reboiling temperature at a given pressure. As a result, the pressure of the gas used to reboil the high purity oxygen increases with increasing oxygen purity and therefore more work is expended in raising the pressure of this gas. Further, when oxygen products of different purity are required, there are increased costs associated with additional pipework and distributors.
The present invention addresses these problems.